212 ALASKAN GLACIER STUDIES
would be exceedingly complex, gravel in the main, but 'with local lake sediment areas, with patches of moraine, with included plant remains and peat bogs, with alternate layers of two or more of these, and, in places, with gradations to marginal valleys and even rock gorges, where the lateral streams had been forced to cut across rock spurs. All these varieties of conditions are present along the margins of the existing glaciers, and since they have been fully described and discussed elsewhere,1 they will not be further considered here.
Origin and Effects of Icebergs. Origin and Nature. Vast quantities of ice are discharged from the three tidal glaciers,óNunatak, Turner, and Hubbard; and the amount of floating ice in the inlet, particularly in Disenchantment Bay, is very great. The larger portion of this ice is in small pieces, but there are also many large icebergs, some rising 40 to 50 feet out of the water, or even more, and having a length of 200 to 300 feet. Many of the icebergs, especially the smaller, are free from dSbris, but large numbers bear some moraine, while icebergs black with included debris are by no means uncommon. We estimate that about twenty per cent of the icebergs cany a noticeable amount of mo-rainic material, and at all times thousands of tons of debris are being floated in the waters of Yakutat Bay. Naturally most of it is finer material, but many icebergs are seen in Tihich good-sized bowlders are embedded. Since there is little moraine in the portion of the glaciers above water level, and since the marginal moraine-covered portions of the glaciers discharge few icebergs, we assume from the abundance of debris-charged icebergs that there is much moraine incorporated in the lower layers of the glaciers, and that these d6bris-charged layers extend a considerable distance above the bottom of the glaciers.
The discharge of icebergs is almost constantly in progress-from the front of the largest tidal glacier, the Hubbard, and at frequent intervals from the other two. One need look at the Hubbard front but a few minutes to see the fall of icebergs, wnile the sound of their discharge, and the waves to which they give rise, are almost incessant. The most noticeable discharge is that from the ice cliff above water level, from which single blocks and avalanches of many blocks are commonly seen cascading down the ice front and sending the spray high in the air. Occasionally great masses tumble down into the fiord, but most of the discharge is in the form of small pieces, a few feet in diameter. Even when a large mass starts, it usually crumbles in its descent and reaches the fiord in small pieces, for the upper ice is evidently not only brittle, but weakened by much breaking. Often, when viewed from a distance, the falls of ice down the glacier front resemble a mass of falling water. These iceberg falls from the tidal cliff are made possible by the attack of the sea water at and below the visible cliff base, by the extensive crevassing which breaks the upper glacier, and by the melting of the glacier in the air, by means of which the disruption of the ice is extended. By these means the rapidly-moving glacier is checked in its advance.
Work Performed. Because of the rapid and abundant discharge of ice from the cliff above tide level a submerged, projecting ice foot is produced from which masses rise every now and then, producing the larger bergs of the inlet, including the glassy and most of the debris-charged icebergs. Probably most of these rising icebergs are small, but both Russell and the senior author have witnessed the rising of large masses, and there are
i See Tarr, R. S., The Yakutat Bay Region, Alaska, Professional Paper 64, TJ. S. Geol. Surrey, 1909, pp. Oft-106, 120-137; Some Phenomena of the Glacier Margins in the Yakutat Bay Region, Alaska, Zeitschrift ftlr Gletscherkunde, Vol. HE, 1908, pp. 81-110; von Engeln, O. D. Zeitschrift ftlr Gletscherkunde, Vol. VI, 1911, pp. 104-150.